1. Field of the Invention
The present invention relates to a semiconductor device including a capacitor.
2. Description of Related Art
A DRAM (Dynamic Random Access Memory) is a semiconductor memory which generally includes a plurality of memory cells each consisting of a single MOS transistor and a single capacitor. Such a DRAM is incorporated in a wide variety of electronic devices and is widespread.
In Japanese Laid-Open Patent Publication No. 2006-339632, disclosure is made about a capacitor which has a lower metal electrode over a semiconductor substrate, a dielectric film on the lower electrode, and an upper electrode which is laminated on and contacted with the dielectric film and which has a stacked film including a polycrystalline layer of a semiconductor belonging to the fourth group.
The above-mentioned document also discloses a capacitor which has a dielectric film and a stacked film which is formed as an upper electrode by a first thin film of a silicon germanium mixture and a second thin film of silicon and which is contacted with the dielectric film. Herein, the capacitor which uses the upper electrode of a semiconductor material contacted directly with the dielectric film will be called a capacitor of an SIM structure.
Thus, using the upper electrode which has a multiple layer structure including a polycrystalline layer of the fourth group semiconductor material makes it possible to realize a stable structure and to suppress a leak current.
However, the capacitor of the SIM structure which has the upper electrode mentioned in the above-mentioned document is disadvantageous in that resistivity of the upper electrode is high.
As to the DRAM, it is very common that the upper electrode of the capacitor included in each memory cell is commonly connected each other among a plurality of memory cells. Therefore, the upper electrode is formed as a plate-like electrode on a chip. Under the circumstances, when the electrical resistivity of a material forming the upper electrode is high, a local voltage fluctuation is often observed transiently on the plate-like electrode in response to a charge/discharge of the capacitor of the selected one of the memory cells. Such a voltage fluctuation results in a noise on reading data stored in the memory cell and bring about a malfunction.